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Full text of "Text Book Of Mechanical Engineering"

Appendix V.

977

machine tools for cutting metals, because when heated by the
frictional resistance of the cut, they approximately return to their
original condition of hardness. But makers generally forbid the
heating of the tool at any time beyond a bright cherry red,
which is about 1500° to 1550° F. Now the higher we can allow
the steel to be heated during forging, so much higher proportion-
ally can it be allowed to overheat when cutting without drawing
the temper, a fact which has led to the introduction by the
Bethlehem Co. of cutting steel made on the Taylor-White
process, where the higher permissible overheat gives increased
value to the speed or the cut as may be desired. In this process
the composition of the steel may be as follows :—

Chromium

Tungsten
I to 6 %

Molybdenum
i to 6 %

Carbon
•85 tO  2%

but the best results are obtainable when there is—

i % Chromium with 4 % Tungsten ;
or? as an alternative,

i% Chromium with 2% Molybdenum;
or, as a further choice,

i% Chromium, with
2% Tungsten, and with
i% Molybdenum,

the proportion of carbon within the limits given being immaterial.
In addition to careful composition, the steel must receive a
special treatment, which consists of raising the temperature (say
during forging) to between 1500° and 1700° F., the surface being
protected with powdered slag, then (after forging) letting down
rapidly, though steadily, by placing in a bath of lead till a
temperature of 1240° F. is reached It is important that not the
least rise of temperature be allowed at this stage, however brief.
The heat should now be suffered to remain at between 700° and
1240° for the space of about five minutes, which may be done
while cutting if desired; and with steel thus made and treated, a
cutting speed of 150 feet per min. is easily practicable. The
outer surface is generally damaged in heating, and should be
ground off.